fix: honor Retry-After on 429s so concurrent retries don't thunder

[vedant7007]

Problem

scripts/lib/retry-interceptor.js backs off by INITIAL_DELAY_MS * 2^(n-1) + up to 200ms jitter and ignores the server's Retry-After header. The sync scripts fire batches of CONCURRENCY_LIMIT = 20; on a 429 all 20 retry within a ~200ms band and re-hammer the API in lockstep (thundering herd), likely re-triggering the limit.

Fix

• Parse Retry-After (RFC 7231 delay-seconds or HTTP-date) → ms; absent/garbage → 0.
• Delay = max(Retry-After, exponentialBackoff) so we never retry sooner than the server asked.
• Widen jitter to a full second to de-correlate the batch.

Test

Added a self-contained check for parseRetryAfter covering seconds, HTTP-date (future clamps forward, past clamps to 0), absent, capitalized, and unparseable headers — all pass. Syntax-checked with node --check.

Closes #380

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